I didn't read the earlier posts on this thread, but assuming the sample data
given is in A1:B3 try this:

In C2: =TIMEVALUE(LEFT(A2,LEN(A2)-4)&":"&MID(A2,LEN(A2)-4,2)&":"&RIGHT(A2,2))
In D2: =TIMEVALUE(LEFT(B2,LEN(B2)-4)&":"&MID(B2,LEN(B2)-4,2)&":"&RIGHT(B2,2))
In E2: =D2-C2

Total difference  in minutes (in F2): =(HOUR(E2)*60)+MINUTE(E2)
Seconds portion of difference (in G2): =SECOND(E2)

Copy formulas down as needed. Hope this helps,